//
// Copyright (c) 2023-2025 Ivica Siladic, Bruno Iljazovic, Korina Simicevic
//
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#ifndef BOOST_MQTT5_ASYNC_MUTEX_HPP
#define BOOST_MQTT5_ASYNC_MUTEX_HPP

#include <boost/mqtt5/detail/async_traits.hpp>

#include <boost/asio/any_completion_handler.hpp>
#include <boost/asio/any_io_executor.hpp>
#include <boost/asio/associated_allocator.hpp>
#include <boost/asio/associated_cancellation_slot.hpp>
#include <boost/asio/async_result.hpp>
#include <boost/asio/bind_cancellation_slot.hpp>
#include <boost/asio/execution.hpp>
#include <boost/asio/require.hpp>
#include <boost/system/error_code.hpp>

#include <deque>

namespace boost::mqtt5::detail {

namespace asio = boost::asio;
using error_code = boost::system::error_code;

class async_mutex {
public:
    using executor_type = asio::any_io_executor;
private:
    using queued_op_t = asio::any_completion_handler<
        void (error_code)
    >;
    using queue_t = std::deque<queued_op_t>;

    // Handler with assigned tracking executor.
    // Objects of this type are type-erased by any_completion_handler
    // and stored in the waiting queue.
    template <typename Handler, typename Executor>
    class tracked_op {
        tracking_type<Handler, Executor> _executor;
        Handler _handler;
    public:
        tracked_op(Handler&& h, const Executor& ex) :
            _executor(tracking_executor(h, ex)), _handler(std::move(h))
        {}

        tracked_op(tracked_op&&) = default;
        tracked_op(const tracked_op&) = delete;

        tracked_op& operator=(tracked_op&&) = default;
        tracked_op& operator=(const tracked_op&) = delete;

        using allocator_type = asio::associated_allocator_t<Handler>;
        allocator_type get_allocator() const noexcept {
            return asio::get_associated_allocator(_handler);
        }

        using cancellation_slot_type =
            asio::associated_cancellation_slot_t<Handler>;
        cancellation_slot_type get_cancellation_slot() const noexcept {
            return asio::get_associated_cancellation_slot(_handler);
        }

        using executor_type = tracking_type<Handler, Executor>;
        executor_type get_executor() const noexcept {
            return _executor;
        }

        void operator()(error_code ec) {
            std::move(_handler)(ec);
        }
    };

    // Per-operation cancellation helper.
    // It is safe to emit the cancellation signal from any thread
    // provided there are no other concurrent calls to the async_mutex.
    // The helper stores queue iterator to operation since the iterator
    // would not be invalidated by other queue operations.
    class cancel_waiting_op {
        queue_t::iterator _ihandler;
    public:
        explicit cancel_waiting_op(queue_t::iterator ih) : _ihandler(ih) {}

        void operator()(asio::cancellation_type_t type) {
            if (type == asio::cancellation_type_t::none)
                return;
            if (*_ihandler) {
                auto h = std::move(*_ihandler);
                auto ex = asio::get_associated_executor(h);
                (asio::require)(ex, asio::execution::blocking.possibly)
                    .execute([h = std::move(h)]() mutable {
                        std::move(h)(asio::error::operation_aborted);
                    });
            }
        }
    };

    bool _locked { false };
    queue_t _waiting;
    executor_type _ex;

public:
    template <typename Executor>
    explicit async_mutex(Executor&& ex) : _ex(std::forward<Executor>(ex)) {}

    async_mutex(const async_mutex&) = delete;
    async_mutex& operator=(const async_mutex&) = delete;

    ~async_mutex() {
        cancel();
    }

    const executor_type& get_executor() const noexcept {
        return _ex;
    }

    bool is_locked() const noexcept {
        return _locked;
    }

    // Schedules mutex for lock operation and return immediately.
    // Calls given completion handler when mutex is locked.
    // It's the responsibility of the completion handler to unlock the mutex.
    template <typename CompletionToken>
    decltype(auto) lock(CompletionToken&& token) noexcept {
        using Signature = void (error_code);

        auto initiation = [] (auto handler, async_mutex& self) {
            self.execute_or_queue(std::move(handler));
        };

        return asio::async_initiate<CompletionToken, Signature>(
            initiation, token, std::ref(*this)
        );
    }

    // Unlocks the mutex. The mutex must be in locked state.
    // Next queued operation, if any, will be executed in a manner
    // equivalent to asio::post.
    void unlock() {
        while (!_waiting.empty()) {
            auto op = std::move(_waiting.front());
            _waiting.pop_front();
            if (!op) continue;
            op.get_cancellation_slot().clear();
            execute_op(std::move(op));
            return;
        }
        _locked = false;
    }

    // Cancels all outstanding operations waiting on the mutex.
    void cancel() {
        while (!_waiting.empty()) {
            auto op = std::move(_waiting.front());
            _waiting.pop_front();
            if (!op) continue;
            op.get_cancellation_slot().clear();
            (asio::require)(_ex, asio::execution::blocking.never)
                .execute([ex = _ex, op = std::move(op)]() mutable {
                    auto opex = asio::get_associated_executor(op, ex);
                    opex.execute(
                        [op = std::move(op)]() mutable {
                            op(asio::error::operation_aborted);
                        }
                    );
                });
        }
    }

private:

    // Schedule operation to `opex` executor using `_ex` executor.
    // The operation is equivalent to asio::post(_ex, op) but
    // for some reason this form of execution is much faster.
    void execute_op(queued_op_t op) {
        (asio::require)(_ex, asio::execution::blocking.never)
            .execute([ex = _ex, op = std::move(op)]() mutable {
                auto opex = asio::get_associated_executor(op, ex);
                opex.execute(
                    [op = std::move(op)]() mutable {
                        op(error_code {});
                    }
                );
            });
    }

    // Executes operation immediately if mutex is not locked
    // or queues it for later execution otherwise. In both cases
    // the operation will be executed in a manner equivalent
    // to asio::post to avoid recursion.
    template <typename Handler>
    void execute_or_queue(Handler&& handler) noexcept {
        tracked_op h { std::move(handler), _ex };
        if (_locked) {
            _waiting.emplace_back(std::move(h));
            auto slot = _waiting.back().get_cancellation_slot();
            if (slot.is_connected())
                slot.template emplace<cancel_waiting_op>(
                    _waiting.end() - 1
                );
        }
        else {
            _locked = true;
            execute_op(queued_op_t { std::move(h) });
        }
    }
};

} // end namespace boost::mqtt5::detail

#endif // !BOOST_MQTT5_ASYNC_MUTEX_HPP
